1. Field of the Invention
The present invention relates to a novel acrylic ester compound which serves as an organic semiconductor material used for an organic electrophotographic photoconductor, an organic EL, an organic TFT and an organic solar cell; more specifically, it relates to an acrylic ester compound and a manufacturing intermediate, and a method for manufacturing an acrylic ester compound, wherein the acrylic ester compound includes a structural unit with a charge transport function, i.e. hole transport property, as well as an acrylic ester or methacrylic ester group in a molecule, and it can form a polymer with high crosslink density by means of a chain reaction.
The present invention relates to a highly abrasion-resistant and highly reliable electrophotographic photoconductor, which may also be hereinafter referred to as a photoconductor or a latent electrostatic image bearing member, as well as an image forming apparatus, an image forming method and a process cartridge, where the electrophotographic photoconductor has an extremely high abrasion resistance; moreover, it has superior electric properties such as charge property, sensitivity and rest potential accumulation property; and it can maintain high image quality with few image defects for a long period of time.
2. Description of the Related Art
An organic semiconductor material having a charge transport function is useful in various applications such as organic electrophotographic photoconductor, organic EL, organic TFT and organic solar cell. As a method for giving the charge transport property to an organic material used for such applications, i.e. resin used as a binder in forming a functional film, a method to mix and disperse a charge transport material in a resin is the most common, and this method is widely adopted for forming an electrophotographic photoconductor.
However, since it is difficult to ensure the mechanical strength and the heat resistance simply by mixing and dispersing a charge transport material in a resin, a material with sufficient properties cannot be achieved. Therefore, it is an effective method to attach and integrate a charge transport material and a resin used as a binder to improve these properties.
Consequently, the integration of a charge transport material with a resin has been recently addressed, and various charge transport monomers having a functional group for chain polymerization, e.g. radically polymerizable functional group, in their charge transport structure and polymers thereof have been proposed.
For example, various charge transport monomers having two or more functional groups for chain polymerization have been proposed, and an application of these to an electrophotographic photoconductor has been proposed. Among charge transport monomers proposals, an acrylic ester compound has a favorable cross-linking property, and many proposals thereof have been made including Japanese Patent Application Laid-Open (JP-A) Nos. 2000-66424 and 2000-206716. These proposals claim that the application of such charge transport monomers can improve the precipitation resistance, the abrasion resistance, the scratch resistance, the sensitivity and the rest potential.
Also, the applicants of the present invention have proposed before an acrylic ester having a triphenylamine moiety and a polymer thereof in Japanese Patent (JP-B) No. 3164426. The use of this acrylic ester can improve the sensitivity and the durability of an electrophotographic photoconductor.
With regard to the hole mobility which indicates the charge transport property, it has been known that an aminobiphenyl structure or amino-substituted stilbene structure with more expanded conjugated system than a simple triphenylamine structure shows high mobility; among the disclosed charge transport monomers, monomers having these structures are particularly useful. The formation of a three-dimensional cross-linking film with sufficiently high crosslink density by means of a chain reaction with such charge transport monomers provides a film with high degree of hardness and high heat resistance as well as scratch resistance. The durability may be improved by using such film for various organic semiconductor devices. However, the increase in the crosslink density reduces the essential charge transport property, and as a result, sufficient functions cannot be obtained.
In other words, many of the proposed charge transport monomers cannot simultaneously satisfy the formation of a structure with high crosslink density which can meet the various disabilities such as abrasion resistance and scratch resistance and the development of a favorable charge transport property. The development of a novel compound which can satisfy the both requirements has been desired.
Also, an organic photoconductor (OPC) has various advantages including: (1) optical properties such as wide light absorption wavelength range and large absorption quantity, (2) electric properties such as high sensitivity and stable charge property, (3) wide selection of materials, (4) the ease of manufacturing, (5) low cost, and (6) non-toxicity, and it has widely been used for a copier, facsimile, a laser printer and a complex machine thereof in place of an inorganic photoconductor.
Recently, the downsizing of an image forming apparatus has promoted the reduction in the diameter of the photoconductor. Combined with the moves towards the speeding-up and the maintenance free of apparatuses, the increased durability of a photoconductor has been highly craved. In this view, an organic photoconductor is generally soft since the charge conductor has a low-molecular charge transport material and an inert polymer, and it is disadvantageous since the mechanical load by a developing system and a cleaning system in an electrophotographic process can easily cause abrasion in a repeated use. Also, the demand for high image quality has reduced the size of toner particles, and the rubber hardness and the contact pressure of a cleaning blade have been increased to improve the cleaning ability. This is also one of the factors which promote the abrasion of a photoconductor. Such abrasion of a photoconductor degrades electric properties such as sensitivity and charge property, and it increases the occurrences of abnormal images such as decrease in image density and background fog. In addition, a scratch with local abrasion results in an image with streaks with insufficient cleaning.
Therefore, improvements in the abrasion resistance of a photoconductor have been examined. For example, JP-A No. 56-48637 discloses a photoconductor in which a crosslinking binder is used in the charge transport layer; JP-A No. 64-1728 discloses a photoconductor which a polymer charge transport material is used; JP-A No. 04-281461 discloses a photoconductor in which an inorganic filler is dispersed in the charge transport layer; JP-B No. 3262488 discloses a photoconductor which includes a polyfunctional acrylic ester monomer hardener; JP-B No. 3194392 discloses a photoconductor having a charge transport layer formed with a coating solution including a monomer with a carbon-carbon double bond, a charge transport agent with carbon-carbon double bond and a binder resin; and JP-A Nos. 2000-66425 and 2004-212959 disclose a photoconductor including a compound obtained by curing a hole transport compound having two or more functional groups for chain polymerization in a molecule.
These modifications improved the abrasion resistance compared to conventional equivalents. A conventional photoconductor had its surface refaced with abrasion even though an adhesion of a foreign substance or a scratch occurred on its surface, and image defect did not persist indefinitely. However, once an adhesion of a foreign substance or a scratch occurs on the surface of a photoconductor with improved abrasion resistance, the condition and image defect persist indefinitely.
Recent requirements for high image quality as well as energy saving has reduced the toner diameter, which has decreased the softening temperature. In order to secure the flowability of the toner, inorganic particles such as silica are added to the toner. There are occasions the silica particles stick in the OPC surface in the developing process. When this happens, the toner component such as wax accumulates around it and inhibits the development, and an image defect with white spot occurs.
The four hundred and several tens of radically polymerizable compounds disclosed in JP-A No. 2004-212959 could not simultaneously satisfy the high abrasion resistance and the electrical property with little occurrence of rest potential at a high level. The reason thereof is the presumably insufficient cross-linking. Various attempts have been made to improve this such as increasing the fraction of the radically polymerizable groups, polyfunctionalizing the radically polymerizable groups and reducing the molecular weight of the charge transport structure. However, there is a limitation in the reduction of the molecular weight of a structure which develops the favorable charge transport property. On the other hand, the polyfunctionalization is effective for increasing the fraction of the content but decreases the charge transport property after curing. The reason for this is not clear, but it is presumably because the heavy cross-linking constrains the molecular movement in the charge transport structure, reduces the space for free movement is reduced and decreases the hopping mobility of the charge.
Therefore, a latent electrostatic image bearing member with sufficiently satisfactory performance and a related technology thereof have not yet been obtained thus far even with the use of a specific radically polymerized compound.